Unravelling the kinetic and thermodynamic drivers of NO variability in ammonia-natural gas combustion based powertrains

The deployment of ammonia–natural gas combustion in clean powertrains is limited not only by combustion instability, but also by the lack of cycle-resolved tools able to connect stochastic combustion behaviour with emissions formation and control. This work addresses this gap by developing an integrated cycle‑resolved digital diagnostic framework combining engine in‑cylinder pressure analysis, a two‑zone thermodynamic model, a digital twin for residual gas estimation, and detailed chemical kinetics to predict cycle‑to‑cycle variability (CCV) of emissions in low- and zero-carbon fuels, including ammonia (NH 3 ), natural gas (NG) and their blends. The results show that the key phenomena governing NO emissions and their variability when NH 3 replaces NG are: i) slower flame kinetics, particularly during early flame development, which limit combustion stability at high NH 3 fractions; and ii) a reduction in residual gas fraction driven by higher specific heat ratios (γ), which enhance blow-down. Consequently, increasing NH 3 substitution reduces NO formation under the investigated conditions, as the effects of lower peak pressures and temperatures dominate over the longer residence times in the burned-gas zone, reduced residual fractions, and the contribution of fuel-bound nitrogen. NO variability is shown to be partly inherited from combustion dynamics, increasing with NH 3 due to greater flame kernel instability. The kinetic analysis further identifies oxygen-sensitive burned-gas pathways controlling NO 2 , N 2 O, CO and CO 2 evolution under stoichiometric operation, providing mechanistic insight into the local nitrogen-species chemistry associated with NH 3 –NG combustion. These findings identify combustion temperature and species residence time as key control-relevant parameters governing emissions formation within the narrow stoichiometric window required for three-way catalyst (TWC) operation. The proposed integrated diagnostic framework provides a practical predictive solution supporting the design and control of NH 3 –NG combustion systems in future clean powertrains.